Calibration Approach and Standards

Calibration Spheres

The availability of light integrating spheres improves our calibration techniques significantly. They allow the field of view of the instrument to be filled with illumination which is uniform over large angles. The illumination in the sphere is monitored with a standardized (NIST) photodiode detector placed behind apertures which define the solid angle of illumination and the area of the detector.

We have found it necessary to illuminate the sphere through a monochrometer to determine the sensitivity at a particular wavelength. Bandpass filters used for this purpose appeared to give erroneous readings, presumably due to "red leaks" at long wavelengths. The diode detector is sensitive to the whole wavelength range from 2500Å to 1.1 µ and had to be open to any radiation in the sphere if the detector calibration was to be used directly. Our most successful and repeatable calibrations were obtained when the sphere was illuminated through a high-speed, low-resolution monochrometer. In order to work in the 2500Å to 4500Å region a xenon arc lamp was required.

Star Calibration

We have found one of the most successful methods of calibration in the field or in flight is the use of stars; GLO has star tracking capabilities. The advantage lies in the selection of stars which have "hot" blackbody radiation curves and therefore are good sources for covering the whole wavelength range of the spectrograph. The stellar spectra are well known and approximate a blackbody continuum at our resolution.


The following is an example of a star calibration from the GLO-1 experiment on STS-53. Figure 1 shows the digitized stellar spectrum of -Pup. Figure 2 shows the spectral response of the GLO spectrographs to this star passing through its field of view. Figure 3 shows the spectra in Figure 2 calibrated with sensitivity curves determined by knowing the spectral content of the stellar source and the sensitivity of the GLO instrument at 5000Å, where the stellar spectrum was normalized.

This star spectrum was very important because it showed us that the reflectivity of the UV channels from 1150Å to 3200Å were degraded before flight. This amounted to an order of magnitude drop in sensitivity at 1200Å. Figure 4shows the calibrated star spectrum at the short wavelength and compared with the stellar spectrum degraded to the GLO resolution. The sensitivity through the noisy area had dropped to near zero, as seen in the actual recorded signal, Figure 2. The noise is amplified by a large factor, but does show interesting correlations. In the future these reflecting surfaces will be kept in a dry nitrogen atmosphere until launch to prevent degradation.

o Last Updated: 11 March, 1996

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Jesus A. Ramirez